The present invention relates to a position correction method carried out in order to realize accurate processing, in processing such as photomask processing used in the manufacture of semiconductor devices using a focused ion beam apparatus, namely a point drift correction method.
When executing a method in which a focused ion beam is irradiated to repeatedly scan over a specified region on a sample surface having a patterned film formed on a substrate surface, the patterned film formed on a substrate surface, the patterned film is removed by etching, and organic compound vapor is sprayed on a specified pattern processing region on the substrate using a gas gun together with forming a patterned film by irradiating an ion beam, accurate positioning of the irradiating ion beam with respect to the sample is necessary. However, when carrying out fine processing using a focused ion beam, problems arise such as the stability of a stage etc. for mounting an ion beam optical system and the sample, and positional drift of the ion beam irradiated on the sample due to variations in environmental conditions such as potential difference and temperature of the sample surface. Conventionally, in order to perform accurate processing, a method has been used in which a pattern suitable for the sample surface is registered and stored as a reference pattern. The reference pattern is then detected at regular intervals during processing and when there is driving from a stored position processing is carried out to correct this positional drift.
The main components of a focused ion beam apparatus performing this ion beam processing are shown in FIG. 5. An ion beam focused through an ion optical system, not shown in the drawing, is suitably deflected by a scanning electrode 1 and irradiated onto a surface of a sample 2 mounted on a sample stage 3. If the ion beam is irradiated onto the sample surface, secondary charged particles are ejected from the sample surface, the nature of the secondary charged particles depending on the sample material at sections irradiated with the ion beam. Secondary charged particles ejected by the irradiating beam are captured by the detector 4 and an amount of charged particles is detected. This value is digitized by the A/D converter 6, and stored in a storage section of the computer 8 as data pertaining to locations subjected to beam irradiation. If the computer designates beam scanning for a specified region, a deflection voltage is applied to the scanning electrode 1 so as to scan the region, via the drive system 7. Detection values of secondary charged particles for each beam spot are stored together with positional information, based on this scanning, to obtain a scanned image of the region designated by the computer 8, thus making it possible to display images as required on the display 10. An operation for removing a patterned film by etching irradiates the processing region with a beam as a result of the computer 8 applying a suitable deflection voltage to the scanning electrode 1 through the drive system 7, based on settings made via the operating section 9. Also, processing for forming the patterned film on the substrate involves the computer executing processing to spray organic compound vapor onto a specified pattern processing region using a gas gun 5 through the drive system 7, and applying a suitable deflection voltage via the drive system 7 to irradiate the specified region with an ion beam, based on settings made via the operating section 9. This type of ion beam processing apparatus performs processing in the above described operation, and also executes operations to acquire the scanned image repeatedly during processing, detect where the reference pattern drifts form the initial position and corrects this drift, so that positional drift does not arise.
Japanese Patent Laid-open No. Hei. 5-4660 discloses a method of performing spot processing using an ion beam and using these spots as basic reference points, in order to improve recognition accuracy of a sample without a suitable reference pattern and a reference pattern. Specifically, as shown in FIG. 4, secondary charged particles are knocked off while scanning a focused ion beam to display a sample surface image. A reference mark (start time) M0 is stored as a position on the image. After scanning a specified number of tomes by limiting a scanning range of the ion beam for the processing operation, a scan image is acquired again, a reference mark (correction time) Mi is detected, an amount of slippage of the reference mark is obtained by comparing with the reference mark position M0 being stored, and finally the beam position recognition is corrected based on this amount of slippage (amount of movement).
A description will now be given of an operation in conventional ion beam processing, for cross sectional observation of a semiconductor element shown in FIG. 4. If the place where it is desired to carry out cross sectional observation of the sample (the finishing processing region in the drawing) is specified, then first of all a reference mark M is etched at a position a suitable distance from the desired location, a protection film is deposited over an area close to the cross section to be observed, large holes are formed in a forward section of the part to be observed, the large holes are made wider, and finally finishing processing is performed to polish the section to be observed. Precise processing is possible as long as the positional relationship between the sample and the ion beam at this time is kept constant, but as has already been mentioned, microscopic positional variations occur during processing. Accordingly, positional correction is carried out in a computer 8 by detecting the slippage by interrupting processing to obtain a scan image and then comparing the reference mark Mi with the initial image position M0, and correcting a voltage applied to a scanning electrode. If the amount of positional correction is large, correction is performed by moving the sample stage 3, but generally speaking drift that occurs during processing is extremely small, and correction is performed by adjusting the scanning electrode. In the related art, this positional correction is generally carried out at fixed intervals, as shown in FIG. 3. In order to carry out accurate processing it is necessary to frequently confirm the position of the reference mark (reference point) M, but if such actions are carried out frequently the amount of time spent on the image processing is increased, and overall processing time becomes long. Shortening the processing time and improving accuracy are therefore mutually incompatible.
The present invention aims to solve the above described problems, and provides a focused ion beam processing method for promoting processing that is capable of very accurate finishing processing without the need for a substantial amount of time for confirming reference points.
The present invention realizes high precision focused ion beam processing in a comparatively short time, with no wasted time, by carrying out positional correction using reference mark confirmation at short time intervals when performing fine processing that requires high accuracy, but carrying out positional correction at long time intervals when performing processing that does not require high accuracy, thus doing away with inefficient positional correction.